Colloidal suspensions, such as paints, blood plasma, and certain foods, typically comprise a plurality of colloidal particles dispersed throughout a continuous liquid or gaseous medium. A particular type of colloidal suspension which has found important application in recent years is that of liquid toner used in electrostatic and electrographic plotters and printers. Liquid toner is typically composed of a liquid solvent, a charge control agent, and a plurality of toner particles dispersed throughout the solvent. The charge control agent is included in the suspension for the purpose of imparting to the toner particles the charge needed by the particles to deposit onto a charged surface such as a statically charged piece of paper. As the liquid toner is used for printing purposes, the toner particles are progressively depleted, leaving behind only the charge control agent and the solvent. Once the concentration of toner particles falls below a certain level, the liquid toner becomes ineffective and must either be replaced or be replenished with additional toner particles. From this discussion, it is clear that the effectiveness and, hence, the quality of the liquid toner is based upon the concentration of the toner particles. For best results, an optimal ratio should be maintained between the concentration of the toner particles and the molecular concentration of the suspension agent including the solvent and the charge control agent.
Since the quality of a liquid toner is mainly a function of the toner particle concentration, an effective method and apparatus for testing the quality of the toner should provide a clear indication of the toner concentration. The prior art, however, fails to provide a method or a means for satisfactorily determining the quality of a liquid toner or a colloidal suspension in general. The prior art teaches a method for measuring the electro-kinetic properties of a colloidal suspension (U.S. Pat. No. 4,497,208), a method for measuring the potential of colloidal particles in a colloidal suspension (U.S. Pat. No. 4,552,019), and a method for measuring the electrical conductivity of a colloidal suspension (U.S. Pat. No. 4,907,453). The measurements of electro-kinetic properties, potentials, and conductivity, however, do not provide a good indication of the toner particle concentration, and hence, fail to give an accurate indication of the quality of a liquid toner. Hence, a need exists for an effective method and apparatus for determining the quality of a liquid toner and a colloidal suspension in general.